This invention relates to amplifiers, and, more specifically, to an amplifier having a flat output characteristic that uses a HEMT (High Electron Mobility Transistor) and/or PHEMT (Pseudomorphic High Electron Mobility Transistor) with feedback circuitry having low electrical input power requirements and suitable for operation at microwave frequencies and above. Such an amplifier is particularly suited to space-borne applications, such as satellites, where weight concerns are generally paramount. Microwave frequencies are generally designated to be in the range from about 1 GHz to about 30 GHz while millimeter-wave frequencies are generally considered to lie in the band from about 30 GHz to about 300 GHz.
In weight sensitive environments, like air-borne and space-borne applications, a significant portion of the overall weight being carried is due to the electronic systems and other items on board that are used to support operation of the electronic systems. For example, in a satellite, batteries or other energy storage medium or combination thereof is required to supply electrical power to the electronics on demand. Further, inasmuch as it is not practical to store large amounts of power because of the number of storage devices and the attendant extra weight required, a regenerator, such as solar cells arranged in a solar panel, may be used to recharge the storage system.
The rate at which the electronics demand electrical power will determine the rate at which the storage system will need to be recharged. In the case of solar cells, the rate of recharging is directly proportional to the number of solar cells, with increasing rates demanding more cells which ultimately determines the size of the solar panels required to hold the cells. As the number of cells increases, the size of the panel and its attendant weight also increase.
The necessity to provide required electrical power to the electronics may also be addressed by appropriate selection of the amount and type of energy storage media to be used. As the number of batteries for storing energy increases, the electrical power available increases but so does the total weight of the energy storage system.
One reason there is such concern about weight with regard to a satellite is that typically satellites are delivered from earth, or from an air-borne or space-borne platform, by a delivery system, like a rocket, which uses thrust engines to provide the desired trajectory for the delivery system and ultimately the satellite. The amount of thrust, and therefore the size of the engines required, is directly proportional to the overall weight of the delivery apparatus which includes the weight of the payload, or satellite. The cost of providing larger or more engines for obtaining additional thrust, increases faster than a straight-line relationship with respect to overall weight, and may even approach or exceed an exponential relationship between cost and available thrust. This relationship may be especially pernicious when applied to the cost to obtain an increase in the marginal thrust beyond some threshold reference.
Previous electronic amplifiers have employed, MESFETs (Metal Semiconductor Field Effect Transistor) as active elements. However, use of MESFETs with feedback has been generally limited for weight sensitive environments to frequencies in the lower portion of the X-band (8-12 GHz) and below, due to inadequate or insufficient individual MESFET device gain.
An article entitled "Ultra-Flat Low-Power Process Insensitive Ku-Band HEMT Feedback MMIC" by D. R. Helms and M. J. Fithian, named inventors hereof, was published in the Proceedings of the IEEE Microwave and Millimeter-Wave Monolithic Circuits Symposium having a date of Jun. 1, 1992 and is hereby incorporated in its entirety by reference thereto.